Numerous solid materials contain valuable and recoverable volatile constituents which may be recovered by retorting. In particular, hydrocarbons and other carbonaceous materials offer a significant source of materials which may be retorted to recover valuable components. There are many abundantly available minerals from which hydrocarbons may be practically recovered by retorting or pyrolysis, including oil shale, oil and tar sands, bituminous and subbituminous coals, coal shale and coal tailings.
Oil shale is a very abundant mineral made up of volatile organic constituents and inorganic minerals. The organic portion, called kerogen, is a high molecular weight polymer cross-linked in three dimensions and insoluble in solvents. The mineral portion of certain shales such as western shales, for example, is about half dolomite and calcite, i.e., carbonates of magnesium and calcium which calcine to MgO and CaO at sufficiently elevated temperatures. The remainder of the mineral matrix usually contains about 10-15% silica and silica alumina compounds containing sodium, potassium or calcium and about 1-5% of iron sulfide. Since the kerogen cannot be dissolved or washed out of the shale with solvents, it is commonly converted to oil, gas and coke by destructive distillation in a shale retort. Thus, crude shale oil and gas may be produced by heating oil shale to about 700.degree. F. or higher. Oil and gas so produced will be typically processed for nitrogen and sulfur compound removal in subsequent upgrading operations. The carbonaceous residue which results from the retorted shale may be burned for heat recovery, but this is not yet commercially common.
The oil shale retorting industry has special characteristics and requirements. To begin with, solids mining and processing rates are enormous. For example, all mining in the United States totalled 3 billion tons in 1976. Recovery of 600,000 barrels per day of oil shale would correspond to mining 0.3 billion tons per year or 10% of all United States' mining, but only 3% of United States' oil consumption. Compared to petroleum refining, high heat loads exist in oil shale retorting because sensible heat must be supplied to the minerals, which amount to about 80 weight percent of the shale for American shales. Complete air-solids contacting is required in burning and the carbon is 150% greater than in a fluid bed catalytic cracker of equivalent oil feed. For example, a 50,000-barrel-per-day shale retorting plant will require as much air and burn as much carbon as a 125,000-barrel-per-day catalytic cracker. High overall thermal efficiency is also important. The residual carbon contains between 10 and 50% of the total Btu's available in raw shale depending upon the origin of the shale. Among other special considerations in retorting oil shale, one must provide an ability to handle rich shale without agglomeration and raw shales with a wide range in kerogen content.
Currently known processes for retorting shale can be classified into four main types according to the method of supplying heat to the retort as reported in Atwood, M. T., "Above-Ground Oil Shale Retorting: The Status of Available Technology", Engineering and Mining Journal, Sept., 1977; and Nowacki, D., Editor "Oil Shale Technical Data Handbook"; Noyes Data Corp., Parkridge, N.J., 1981. In brief, Types 1 and 2 retorts involve heating of shale by gas. As a consequence, the retort off-gas is diluted with flue gas to give a typical 120-Btu-per-cubic-foot gas which is generally not saleable. Examples of the Type 1 retort are the Paraho direct process and the Superior Oil circular-grate process. In a Type 2 retort, recycle retort gas is preheated indirectly to approximately 1200.degree. F. before entry into the retort. Because the recycle gas is undiluted with flue gas, heating values above 800 Btu per cubic feet are attainable. Part of the retort gas is burned to heat the recycle gas. Union Retort Type B and Union Oil SGR-3 processes are examples of a Type 2 retort. The Type 3 retort is characterized by the recirculation of hot ceramic balls to supply retort heat. TOSCO II is the foremost example of this type. Finally, a Type 4 retort is characterized by the recycle of spent shale from the combustor to supply retort heat. Lurgi-Ruhrgas and Chevron processes are examples of this type and they require lift-combustors where the solids are lifted by compressed air which is costly. In view of the brief overview of the above background and prior art in connection with the retorting of solid materials, especially hydrocarbon-containing minerals, for the purpose of recovering volatile components therefrom for isolation and reuse, further improvements are desirable. In substance, there are demands for new apparatus and methods for the retorting of such materials with high efficiencies in order to overcome the disadvantages associated with known techniques and devices.